In German Utility Model (Gebrauchsmusterschrift) No. 7 438 034, there is described a device which can be used in conjunction with a sewing machine and in which a pair of flexible workpieces, e.g. fabric pieces to be stitched together, can be clamped, the device applying a retarding force to the clamp to resist the displacement of the workpieces past the sewing station by the sewing machine feed.
The device comprises a pair of alternately effective clamps adapted to engage the workpieces and connected to a weight which provides the retarding force, i.e. a restoring force resisting the displacement of the clamp which is in engagement with the workpieces toward the sewing station. A cable is connected between the downwardly hanging weight and the clamp so that the weight is raised as the clamp is horizontally displaced toward the sewing station. Upon release of the clamp from the workpieces, the weight again falls to enable return of the clamp to its starting position so that a new pair of workpieces can be mounted in the clamp and the operation repeated.
The weight is intended to provide a light braking force to the advance of the workpieces so that the latter will maintain a linear feed into the sewing machine and a uniform sewing seam can be produced. The device thus performs the function of the sewing machine operator when he retards the advance of the fabric pieces through the sewing station.
The clamp retains the two pieces so that their edges to be stitched together coincide and the weight maintains a slight stretching effect to insure continuous and uniform stitch formation. The traction force is relatively small, e.g. of the order of 50 grams.
It has been found, however, that even such a minimum traction or tension force is too great for certain workpieces, e.g. very thin materials, so that excessive stretching develops which results in detrimental folds in the stitched seam.
In many cases it is desirable to reduce this retarding force to close to zero, a difficult enterprise with conventional systems of the type described, even where the weights are interchangeable. One of the reasons for this problem is that it is desirable to displace the system into its starting position and the restoring force necessary for this purpose is far in excess of the close-to-zero value which may be desired for fabric feed.
In other words, a restoring force which tends to permit the clamp to be advanced toward the stitching location with sufficiently low tension on the workpiece is frequently insufficient to overcome the friction and the inertia of the clamp in displacing it back into its starting position. Naturally, when it is desired to displace the clamp back into its starting position at high speed so that the operation can be repeated, immediately following the conclusion of a previous stitching operation, as is necessary for modern sewing machine production rates, the restoring force is inadequate. The conventional system described has thus been incapable of solving the problem, especially when it is desirable to operate with a single fabric clamp.